It's alleged the money was earmarked as "speaker's fees", but prosecuters say the fees were actually kickbacks from drugmaker Insys Theapeutics.

The events the doctors were speaking at were just social gatherings, they say.

"These prominent doctors swore a solemn oath to place their patients' care above all else," said Geoffrey Berman, US attorney for the Southern District of New York. "Instead, they engaged in a malignant scheme to prescribe fentanyl, a dangerous and potentially fatal narcotic 50 to 100 times more potent than morphine, in exchange for bribes in the form of speaker fees."

Insys founder and former chief executive John Kapoor is already facing charges over dodgy fentanyl prescriptions the New York Times reports.